1. Field of the Invention
The present invention relates to an optical module having a semiconductor light-emitting or -detecting device and a lens which are centrally aligned and held by a resin housing, and more particularly to an optical module in which a lens is fixedly held by lens grip fingers and a lens seat which are integrally molded with a resin housing.
2. Description of the Prior Art
Optical modules have a semiconductor device, such as a semiconductor light-emitting device such as a semiconductor laser or the like, or a semiconductor light-detecting device such as a photodiode, and a lens which are held in central alignment with each other, and are used in the field of optical communications. For example, a computer system used for data communications has a semiconductor light-emitting device module and a semiconductor light-detecting device module which are installed as a module pair on a board. An optical module comprises a semiconductor device, a lens, and a housing which holds the semiconductor device and the lens and also fittingly holds the ferrule of an optical plug from the party to communicate with. When the optical plug is connected to the optical module, the semiconductor device is optically coupled through the lens to an optical fiber held by the ferrule. The housing generally comprises a holder which holds the semiconductor device and the lens and a receptacle which fittingly holds the ferrule. The holder and the receptacle are manufactured as separate members and fixed in central alignment with each other.
The lens incorporated in an optical module generally comprises a spherical lens or a gradient-index rod lens. The lens may also comprise any of various other lenses. Of those different lenses, the spherical lens is widely used because it is inexpensive as it can easily be manufactured highly accurately by mechanical machining only and it is easy to assemble in position as it has no directivity at all and does not need to be adjusted in orientation for installation. The lens may be fixed to the holder in any of various different ways. According to one process, the lens is put into and positioned in a recess in the holder (lens mount), after which a resin-base adhesive is coated around the lens and then thermoset. According to another process, a molded glass ring of low melting point which is slightly greater than the outside diameter of the lens is placed around the lens, and then melted with heat.
In the former process, since the liquid resin-base adhesive needs to be poured into a small region around the lens, it cannot easily be handled before thermoset. Inasmuch as the holder is usually made of a metal material, the bonded surface of the lens may crack with heat due to the difference between the coefficients of thermal expansion of the lens and the holder.
The latter process also suffers disadvantages in that the glass of low melting point tends to be devitrified and the bonding strength tends to be lowered when left to stand at high temperatures, and the cost is high. Because the molded glass ring of low melting point is produced by molding a powder of glass of low melting point under pressure, when the molded glass ring is placed in position, it scatters fine fragments and particles, which are applied to the surface of the glass. If the molded glass ring is melted with heat in order to fix the lens in place, the fine fragments and particles applied to the surface of the glass are also melted, and the melted glass itself flows around the lens, producing a localized film of glass on the surface of the lens. The glass of low melting point is not resistant to humidity, and is liable to be devitrified with time. If the glass on the surface of the lens is devitrified, then it will cause a reduction in the intensity of light passing through the lens. When exposed to humidity, fine cracks are developed in the glass on the lens, making the glass brittle. Therefore, the bonding strength of the glass is lowered possibly to the point where the lens may be dislodged. This shortcoming could be avoided by employing a countermeasure to make the glass resistant to humidity. Such a countermeasure, together with the fact that the molded glass ring of low melting point is relatively expensive, increases the cost of the optical module.